This invention relates to a process and apparatus for continuously feeding and uniformly dispersing solids in molten metal.
The desire to uniformly disperse solid additives in molten metal arises from the need to perform a variety of functions which such solids are capable of performing in the refining of metal. Such functions may include deoxidation, desulfurization, degassing, alloying and fluxing. For example, calcium containing material, in granular or powdered form, is added to molten steel to react with oxygen and sulfur and/or to modify the shape of inclusions, thereby improving the physical properties of the steel. Lime, CaC.sub.2 or magnesium containing material are added to blast furnace iron to desulfurize the melt. In addition, during the production of steel, it is customary to adjust melt chemistry following decarburization by the addition of alloying ingredients to make the metal meet specifications.
The simplest and most common method for making such solids additions is by simply shoveling the solids into the vessel as it is being filled with the melt. One of the major difficulties encountered in treating metals with solid additives in a vessel stems from the fact that relatively small quantities of solids (for example, 100 lbs.) are normally added to very large quantities of metal (10-100 tons). In large ladles particularly, this disparity in proportions renders uniform distribution of the additive throughout the melt difficult.
A second problem encountered in dispersing solids in molten metal is one of providing sufficiently long contact time between the additive and the metal to permit the necessary heat and mass transfer to occur so that solution and reaction of the additive with the impurities in the metal may take place. Since the density of some solid additives (for example: aluminum, some ferro-silicons, calcium, lime, and magnesium) is much lower than the density of the molten metal, contact time is short due to the high buoyancy of the additive. Consequently, a significant portion of the additive may rise, and end up in the slag before it has had an opportunity to perform its intended function in the melt. This results in inefficient utilization of the additive.
A third problem occurs if the additive is highly volatile. In such case, the additive vaporizes rapidly upon coming in contact with the molten metal, and the resulting vapor bubbles have an even higher tendency to rise and escape from the metal than do solid additives. This problem is particularly severe with elements such as calcium which have low solubility in steel. With short contact times, the solution of vaporized additives requires a high vapor-melt contact area as well as high pressure. There are only limited possibilities for controlling the contact area, while the requirement of high pressure can, as a practical matter, only be satisfied by injection of the additive deep into the body of the melt.
In an attempt to solve the above-mentioned problems, the prior art has proposed a variety of techniques for feeding additives under the surface of the melt. Such methods include: introducing the solid additive into the melt in the form of a wire, shooting the solids into the metal in the form of bullets, and feeding powdery or granular additives deep into the melt through a submerged pneumatic lance. The use of the wire or bullet technique is, however, restricted to metallic additives. The use of submerged lances is subject to operational difficulties such as the formation of skulls at the slag level, excessive metal splashing and vibration of the lance caused by intensive gas bubbling. Furthermore, since pneumatic injection of solids into the metal is a batch process, it has two inherent disadvantages. First, it is necessary to superheat the melt to compensate for the heat loss during the period of additive injection; and second, the metal, if sensitive to oxidation, has to be protected from recontamination by air during teeming.